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No functional change.
Avoid a tricky line in shelter_storm()

kf = (kf == FILE_A) ? kf++ : ....

is tricky becuase kf is updated twice and it happens
to do the right thing just by accident.

Rewrite in a better way.

Spotted by pdimov

No functional change.
Prune negative SEE moves also in PV nodes

This patch is actually the sum of two contributions that
have been tested independently:

1) Pruning of negative SEE moves in PV

After 10000 games at 20+0.05
ELO: 5.18 +-7 (95%) LOS: 99.2%
Total: 10000 W: 1952 L: 1803 D: 6245

2) Remove of bestValue > VALUE_MATED_IN_MAX_PLY condition

After 23000 games at 20+0.05
ELO: 1.63 +-4 (95%) LOS: 88.1%
Total: 23000 W: 4232 L: 4124 D: 14644

The whole patch as been re-tested at long TC with positive results:

After 10000 games at 60+0.05
ELO: 4.31 +-7 (95%) LOS: 98.3%
Total: 10000 W: 1765 L: 1641 D: 6594
Remove pruning condition on alpha

Further simplifying on Lucas's idea, seems reliable
in tests:

ELO: 2.15 +-7 (95%) LOS: 84.9%
Total: 9999 W: 1831 L: 1769 D: 6399
Merge Lucas's "SEE pruning at PV nodes"

bench: 4922272
Merge remote-tracking branch 'upstream/master' into pinned_null